Mapping User Data onto a Time-Frequency Resource Grid in a Coordinated Multi-Point Wireless Communication System

ABSTRACT

Methods and apparatus are disclosed for receiving user data in a wireless communication system that employs coordinated multi-point transmission of the user data from a first cell serving a wireless terminal and a second cell site neighboring the first cell site. In an exemplary system, the first cell site maps control signals and user data to a time-frequency resources according to a first mapping pattern, while the second cell site maps control data and traffic data to the time-frequency resources according to a second mapping pattern. An exemplary method comprises extracting user data, according to the first mapping pattern, from time-frequency resources of a first transmission for the wireless terminal transmitted from the first cell site; detecting a control element transmitted by one of the first and second cell sites, the control element indicating that user data associated with the control element is mapped to the time-frequency resources according to the second mapping pattern; and, responsive to said detecting, extracting user data according to the second mapping pattern from time-frequency resources of a second transmission for the wireless terminal transmitted from the second cell site.

RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 14/341,941, filed 28 Jul. 2014, which is a continuation applicationof U.S. application Ser. No. 13/148,169, filed 5 Aug. 2011, which is aNational Phase of International Application PCT/SE2009/051045 filed 21Sep. 2009, which in turn claims priority to U.S. Provisional ApplicationNo. 61/151,293, filed 10 Feb. 2009. The Applicants incorporate thedisclosures of each of these references in their entireties by referenceherein.

TECHNICAL FIELD

The present invention generally relates to wireless telecommunicationsystems, and more particularly relates to the mapping of user data ontoan Orthogonal Frequency-Division Multiplexing (OFDM) resource grid in awireless communication system utilizing coordinated multi-pointtransmissions.

BACKGROUND

In the so-called Long-Term Evolution (LTE) systems currently underdevelopment by members of the 3^(rd)-Generation Partnership Project(3GPP), downlink transmissions are specified according to an OrthogonalFrequency-Division Multiple Access (OFDMA) scheme. Hence, the availablephysical resources in the downlink are divided into a time-frequencygrid. Generally speaking, the time dimension of the downlink physicalresource allocated to a particular base station (an Evolved Node B, oreNodeB, in 3GPP terminology) is divided into subframes of onemillisecond each; each subframe includes a number of OFDM symbols. For anormal cyclic prefix length, suitable for use in environments wheremultipath dispersion is not expected to be extremely severe, a subframeconsists of fourteen OFDM symbols. A subframe consists of twelve OFDMsymbols if an extended cyclic prefix is used. In the frequency domain,the physical resources allocated to a given eNodeB are divided intoadjacent OFDM subcarriers, at a spacing of fifteen kilohertz, with theprecise number of subcarriers varying according to the allocated systembandwidth. For purposes of scheduling resources (i.e., allocatingresources for use by a given mobile station), the downlinktime-frequency resources are referenced in units called “resourceblocks” (RBs); each resource block spans twelve adjacent subcarriers andone-half of one subframe. The term “resource block pair” refers to twoconsecutive resource blocks, i.e., occupying an entire one-millisecondsubframe.

The smallest element of the LTE time-frequency grid, i.e., onesubcarrier of one OFDM symbol, is called a resource element. There areseveral different types of resource elements, including resourceelements used as reference signals (RS) as well as resource elements forcarrying data symbols (e.g., coded information carrying symbols). Thereference signals enable channel estimation, which can be used forcoherent demodulation of the received signals and may also be used forvarious measurements. Each reference signal defines a so-called antennaport—since a specific RS is used for each port, a given antenna port isvisible to mobile stations (user equipment, or UEs, in 3GPP terminology)as a separate channel. However, an antenna port is a logical entity thatmay or may not correspond to a single physical antenna. Thus, when anantenna port corresponds to multiple physical antennas, the samereference signal is transmitted from all of the physical antennas.

Cell-specific reference signals (also known as common reference signals)as well as UE-specific reference signals (user-equipment-specificreference signals, also known as dedicated reference signals) aresupported in the current LTE specifications. At a given eNodeB, either1, 2, or 4 cell-specific reference signals may be configured. However,only one UE-specific reference signal is available under the currentspecifications.

FIG. 1 illustrates a portion of the LTE time-frequency grid for thecases of 1, 2, and 4 cell-specific antenna ports (which may correspond,for example, to eNodeB's using 1, 2, and 4 transmit antennas,respectively). More particularly, FIG. 1 illustrates a resource blockpair, i.e., twelve subcarriers over a single subframe, for each antennaport. The structure illustrated in FIG. 1 is generally repeated over theentire system bandwidth.

In FIG. 1, reference symbols 110 are high-lighted in the illustration ofthe resource block pair for the case of antenna port 1. Other referencesymbols, for the additional antenna ports, are shaded but nothigh-lighted in each of the various grids. Thus, as can be seen, thereference signals for the different antenna ports are carried in OFDMsymbols 0, 4, 7, and 11 (i.e., the first and fifth symbols of each ofthe two slots in the subframe), for one and two antenna ports. Thefour-port case includes additional reference symbols in OFDM symbols 1and 8 as well.

At any given eNodeB, the actual resource grid may look slightlydifferent from what is illustrated in FIG. 1 in that thereference-signal pattern may be shifted in frequency by an integernumber of subcarriers. The specific shift depends on the cell identifier(ID); the number of unique shifts available depends on the number ofcell-specific antenna ports that are configured. A close examination ofFIG. 1 will reveal that there are six shifts yielding unique referencesymbol patterns in the case of one cell-specific antenna port.Configurations for two and four cell-specific antenna ports will eachsupport three different shifts, as there is, in these cases, a3-sub-carrier frequency shift between reference symbols of differentantenna ports.

Such frequency shifts serve at least two purposes. First, they enablemore effective power boosting of resource elements used for referencesignals, since these resource elements for adjacent cells are lesslikely to collide. Secondly, for purposes of channel-qualitymeasurements, shifting allows the inter-cell interference to be measuredfor the reference-signal resource elements. Since the so-obtainedinterference is a mixture of reference-signal interference and datainterference from other cells, such measurements thus take into accountthe load of interfering cells, at least to some extent.

As previously mentioned, UE-specific reference signals are alsosupported in the current LTE specifications. The pattern for aUE-specific reference is illustrated in FIG. 2, which also illustratesadditional details of the layout of a resource block pair. As seen inFIG. 2, a resource-block pair comprises atwelve-subcarrier-by-fourteen-symbol grid of resource elements 220 (forthe case of a normal length cyclic prefix), or two resource blockstogether occupying a subframe 210. The subframe 210 in turn comprises aneven-numbered slot 212 and an odd-numbered slot 214. The first one, two,three, or four symbols of the subframe are used for a control-channelregion 240 (which may carry one or multiple Physical Downlink ControlChannels, or PDCCHs); the resource block illustrated in FIG. 2 isconfigured with two symbols dedicated to the control-channel region 240.UE-specific reference symbols 230 are also illustrated in FIG. 2; thesereference symbols appear in OFDM symbols 3, 6, 9, and 12. TheUE-specific reference signal effectively defines a fifth antenna port.

The UE-specific reference signal is only associated with thoseresource-block pairs allocated for a particular Physical Downlink SharedChannel (PDSCH) transmission that relies on such reference signals(i.e., those transmissions that are mapped to antenna port 5). Thus, thereference symbols corresponding to a UE-specific reference signal arenot necessarily transmitted in every subframe, or for all resource blockpairs within one subframe. Unlike the cell-specific reference signals,precoding may be applied to UE-specific reference signals in the samemanner as it may be applied to data-carrying resource elements. Thismakes such precoding effectively invisible to the mobile station, in thesense that any precoding will effectively be included in the channelestimates derived by means of the UE-specific reference signals.UE-specific reference signals thus provide enhanced flexibility inmapping a data transmission to different antenna configurations. Inparticular, the use of UE-specific reference signals facilitates themapping of a particular downlink transmission to antennas spread outover different sites.

Data over the PDSCH is transmitted to a given mobile station usingresource elements that correspond to the resource block pairs allocatedto that mobile station for a given subframe. The particular resourceblock pairs involved in the transmission are dynamically selected andsignaled to the mobile station as part of the resource-allocationcontent of the associated control channel, PDCCH, transmitted in thecontrol-channel region of the subframe. As is apparent from FIGS. 1 and2, some of the OFDM symbols outside of the control-channel region areused to carry reference symbols; hence, not all resource elements inthat portion of the resource block pair can be used for PDSCHtransmission. In other words, the mapping of PDSCH onto the resourcegrid is affected by the positions of the cell-specific referencesymbols.

In a classical cellular deployment, the intended service area is coveredby several cell sites at different geographical positions. Each site hasone or more antennas servicing an area around the site. Often, a cellsite is further subdivided into multiple sectors, where perhaps the mostcommon case is to use three 120-degree-wide sectors. Such a scenario isillustrated in FIG. 3. Each sector forms a cell, and a base stationassociated with that cell is controlling and communicating with themobile stations within that cell. In a conventional system, thescheduling and transmissions to mobile stations and reception from themobile stations are to a large degree independent from one cell toanother.

Differing simultaneous transmissions on the same frequencies indifferent cells close to each other will naturally interfere with eachother and thus lower the quality of the reception of the differenttransmissions at a receiving mobile terminal. Interference is a majorobstacle in cellular networks and is primarily controlled inconventional deployment scenarios by planning the network carefully,placing the sites at appropriate locations, tilting the antennas etc.

Performing independent scheduling between different cells has theadvantages of being simple and requiring relatively modest communicationcapabilities between different sites. On the other hand, the cellsaffect each other in that signals originating from one cell are seen asinterference in nearby cells. This indicates that there are potentialbenefits in coordinating the transmissions from nearby cells. In variouscellular systems, separating transmissions in frequency and/or timebetween neighboring cell sites is commonly used to reduce interference.However, this separation has historically been statically configured.More recently, separation in the spatial domain, e.g., by means ofadvanced multi-antenna transmission schemes, has also been widelyexploited, and coordination of neighboring transmissions in the time,frequency, and spatial domains has been proposed to mitigateinterference. Such coordination has recently received substantialinterest in both academic literature and standardization of new wirelesstechnologies. In fact, so-called coordinated multi-point transmission(COMP), see 3GPP TR 36.814 v0.3.2 (R1-090929) is considered one of thekey technology components for the upcoming release 10 of LTE(LTE-Advanced).

COMP may be classified into two separate but related technologies:coordinated scheduling and joint transmission, respectively. In theformer case, the transmission to a given mobile station originates at asingle cell site or sector at a time, while in the latter case multiplesites and/or sectors are simultaneously involved in the transmission.Thus, for example, several cell sites covering a group of cells, such asthe group of seven circles inside the circle of FIG. 3, may coordinatetheir transmissions; a group of cells involved in such a coordination ishere referred to as a COMP cluster.

Obviously, coordination between cell sites requires communicationbetween the sites. This can take many forms and the requirements on datarates and latency for such inter-site communication are to a largeextent dependent on the exact coordination scheme being used.

Apart from the potential problem of site-to-site communicationcapability, coordination exploiting time and frequency is easilyachieved for OFDM systems like LTE by using the normal dynamic resourceallocation feature, which selects the particular resource-block pairsfor transmitting the PDSCH to a given mobile station in a givensubframe. Spatial coordination, on the other hand, involves utilizingmultiple antennas for the transmission; this can include transmissionfrom antennas at geographically distinct cell sites. By modeling thesignals as vector-valued signals and applying appropriate complex-valuedmatrix weights among the transmitting antennas, the transmission can befocused in the direction (in physical space or in a more abstract vectorspace) of the mobile station, while minimizing the interference to othermobile stations. This approach increases thesignal-to-noise-plus-interference ratio (SINR) at the mobile station,and ultimately improves the overall performance of the system.

As previously indicated, the mapping of PDSCH onto resource elements inthe LTE time-frequency grid may vary from one cell to another, even ifthe same resource blocks are used for the PDSCH. One reason is the useof different reference-signal frequency shifts for the cell-specificreference signals. Another reason is that the number of OFDM symbolsused for control signaling can vary dynamically from 1 up to the 4 firstOFDM symbols and may be different for neighbor cells. Hence, theparticular serving cell to which a given mobile station is attachedaffects the mapping of PDSCH to resource elements in the time-frequencyresource grid, as this mapping is intended to be compatible with howother resources such as the reference signals and PDCCH are allocated inthat particular cell. This may create problems for coordinatedmulti-point transmission, where certain transmissions to a mobilestation need to be performed from sites/sectors other than the serving(logical) cell, whether simultaneously or as part of a coordinatedschedule.

SUMMARY

Some of these problems may be mitigated, in some embodiments of thepresent invention, by enabling transmission of PDSCH (possibly includingassociated UE-specific reference signals) according to a resourcemapping that is compatible with the mapping used in a cell other thanthe serving cell. Specifically, in these embodiments it is possible touse a mapping of PDSCH data symbols to the LTE time-frequency gridaccording to a pattern that corresponds to a reference-signal frequencyshift different than that used by the serving cell (i.e., the cell towhich the PDSCH is associated). In some embodiments, the mapping of thePDSCH data symbols to the LTE time-frequency grid may also be adjustedto accommodate a differently-sized control channel (e.g., to accommodatethe fact that a neighboring cell uses three OFDM symbols for PDCCH,while the serving cell uses only two symbols).

In support of this approach, appropriate signaling may be added, in someembodiments, to support dynamic adaptation of the mentioned PDSCHmapping. More specifically, signaling may be added to inform a receivingmobile station about the mapping used for a particular PDSCHtransmission. In other words, this additional signaling informs themobile station of which of several possible PDSCH mappings that themobile station should use when extracting PDSCH data symbols from theOFDM time-frequency grid and decoding the PDSCH. In some embodiments,this additional signaling could be part of the PDCCH, such as the partof the PDCCH carrying scheduling information for the mobile station.

Thus, embodiments of the invention include methods, such as may beimplemented in a mobile station, for receiving user data in a wirelesscommunication system that employs coordinated multi-point transmissionof the user data from a first cell serving the wireless terminal and asecond cell site neighboring the first cell site. In this system, thefirst cell site maps control signals and user data to a time-frequencyresources according to a first mapping pattern, while the second cellsite maps control data and traffic data to the time-frequency resourcesaccording to a second mapping pattern. The control signals may includecommon reference signals, UE-specific reference signals, synchronizationsignals, and the like.

An exemplary method includes extracting user data, according to thefirst mapping pattern, from time-frequency resources of a firsttransmission for the mobile station transmitted from the first cellsite, detecting a control element transmitted by one of the first andsecond cell sites, the control element indicating that user dataassociated with the control element is mapped to the time-frequencyresources according to the second mapping pattern, and, responsive tosaid detecting, extracting user data according to the second mappingpattern from time-frequency resources of a second transmission for thewireless terminal transmitted from the second cell site. In someembodiments, detecting the control element comprises decoding one ormore bits of a received downlink resource allocation message. In some ofthese and other embodiments, the control element indicates one of aplurality of pre-determined shift patterns for common reference signalsinterspersed among time-frequency resources mapped to user data. Instill other embodiments, the control element further indicates that thesecond mapping pattern maps user data to one or more fewer OFDM symbolsthan the first mapping pattern.

The techniques described herein may be applied to coordinatedmulti-point transmission using only coordinated scheduling, such aswhere the first and second transmissions discussed above are transmittedduring first and second non-coincident transmission time intervals.Further, these techniques may be applied where the first and secondtransmissions are at least partially overlapping in time, in which casethe methods summarized above may further comprise separating the firstand second transmissions using one of space-time diversity processing orspatial de-multiplexing processing. In some embodiments, the controlsignals comprise user equipment-specific reference signals interspersedamong time-frequency resources mapped to user data, in which case themethods summarized above may further comprise extracting the userequipment-specific reference signals from the second transmissionaccording to the second mapping pattern.

Other embodiments include various wireless terminals, adapted for use ina wireless communication system employing coordinated multipointtransmission of user data, the wireless terminals including a receivercircuit configured to carry out one or more of the inventive techniquessummarized above and disclosed in detail below. Further embodimentsinclude a transmitting node for use in a first cell site in a wirelesscommunication system using coordinated multi-point transmission of data,where the transmitting node includes a transmitter circuit configured totransmit a control element indicating that user data associated with thecontrol element is mapped to the time-frequency resources according to aparticular pre-determined mapping pattern. Methods corresponding to thistransmitting node are also disclosed.

Of course, the present invention may be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. Upon reading the following descriptionand viewing the attached drawings, the skilled practitioner willrecognize that the described embodiments are illustrative and notrestrictive, and that all changes coming within the meaning andequivalency range of the appended claims are intended to be embracedtherein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the Long-Term Evolution (LTE) time-frequency resourcegrid when a normal cyclic prefix is used. The cases of one, two and fourantenna ports are illustrated.

FIG. 2 illustrates a UE-specific reference signal in a resource blockpair.

FIG. 3 illustrates an exemplary cellular network with three-sectorsites.

FIGS. 4A and 4B illustrate transmissions from first and second cellsites to a mobile station in an LTE system utilizing coordinatedmulti-point transmission.

FIG. 5 is a process flow diagram illustrating an exemplary method forreceiving user data in a wireless communication system employingcoordinated multi-point transmission.

FIG. 6 is a schematic illustration of components of a wirelesscommunication system according to some embodiments of the invention.

FIG. 7 is a block diagram illustrating functional components of anexemplary processing circuit configured according to some embodiments ofthe invention.

DETAILED DESCRIPTION

Although aspects of the present invention are described herein in thecontext of a Long-Term Evolution (LTE) system, as specified by the3^(rd)-Generation Partnership Project (3GPP), those skilled in the artwill appreciate that the inventive techniques disclosed herein may beused in other wireless systems. Thus, although terminology from the 3GPPLTE specifications is used throughout the disclosure, this terminologyshould not be seen as limiting the scope of the invention to only theaforementioned system. Other wireless systems, including WCDMA, WiMax,UMB and GSM, may also be adapted to exploit the techniques describedbelow. Indeed, it should also be noted that the use of terms such asbase station, eNodeB, mobile station, and UE should also be consideringnon-limiting in the sense that their use herein need not imply that thepresent inventive techniques are only applicable in systems employingthe hierarchical architecture of the 3GPP LTE system. Thus, when themethods and apparatus of the present disclosure are adapted to othersystems, the apparatus referred to herein as “base station” or eNodeBmay correspond to a user device or other wireless terminal, e.g.,“device 1”, while the apparatus described herein as a “mobile station”or “UE” may refer to another wireless terminal, e.g., “device 2,” withthese two devices communicating with each other over any suitable radiochannel.

Nevertheless, the inventive techniques of the present invention are mostreadily understood with reference to an LTE system utilizing coordinatedmulti-point (COMP) transmission, where user data is transmitted to amobile station from a first cell site (e.g., a first eNodeB) that servesthe mobile station as well as from a second cell site that neighbors thefirst cell site. As discussed above, the first cell site may map controlsignals, especially reference signals, to the LTE time-frequencyresource grid according to a first mapping pattern, while the secondcell site maps control signals to the resource grid according to asecond mapping pattern, differing from the first mapping pattern.

For example, consider a scenario involving a mobile terminal served by a“logical” cell A, which is normally associated with physicaltransmissions from cell -sector AA, which in turn corresponds to aparticular antenna or array of antennas at a first cell site. Sometimesa PDSCH associated with this logical cell A needs to be transmitted forat least one subframe from cell-sector BB, at a second cell site, whichnormally is associated with a logical cell B. However, cell sector BB isalso transmitting a broadcast channel, BCH, corresponding to logicalcell B. Thus, for at least this one subframe, sector BB transmitssignals for mobile stations being served by logical cell B as well asthe coordinated signal transmission associated with logical cell A.However, if logical cell A is using a different reference signal shiftthan logical cell B (because transmissions from logical cell A arenormally mapped to a physical cell site neighboring the cell sitecarrying cell B transmissions), then sector BB will on some resourceelements carry transmissions both for common reference signals forlogical cell B and PDSCH transmissions associated with logical cell A.Thus, there is a data-to-reference-signal collision problem, causinghigh interference.

In particular, there are a number of different, albeit related,interference scenarios arising from the coordinated transmission. First,because both sector AA and sector BB are transmitting PDSCH according tological cell A's mapping, at least in some resource block pairs, thenstrong interference from the logical cell A PDSCH signals is seen on thecell-specific reference signals for logical cell B. Especially forcell-center mobile stations, this can be a big problem. Normally, amobile station near a sector transmitter enjoys high SINRs, and cantherefore sustain very high data rates. However, with stronginterference to logical cell B's cell-specific reference signals, causedby a coordinated PDSCH transmission corresponding to logical cell A, thechannel estimation accuracy in a cell-center mobile station is likely tobe significantly impaired.

Conversely, the transmission from cell-sector BB of cell-specificreference signals for logical cell B interferes with the coordinatedtransmission of PDSCH for logical cell A. Compared to the former case,this may be somewhat less of a problem, since the logical cell A PDSCHtransmission is likely to serve a mobile station that is not too farfrom the border between the physical areas normally covered bycell-sectors AA and BB. (Otherwise, the mobile station would likely beassociated with logical cell B.) Thus, the interference from logicalcell B reference signals is not worse than the inter-cell interferencenormally encountered in a classical cellular deployment. However, oneimportant goal of coordinated multi-point transmissions is to avoid lowSINRs at the cell edge; this interference mechanism works against thisgoal.

Therefore, it is an object of some embodiments of the invention tomitigate the problems described above. This is achieved by enablingtransmission of PDSCH (possibly including associated UE-specificreference signals) according to a resource mapping that is compatiblewith the mapping used in a cell other than the serving cell.Specifically, it should be possible to use a mapping of PDSCH datasymbols to the LTE time-frequency grid according to a pattern thatcorresponds to a reference-signal frequency shift different than thatused by the serving cell (i.e., the cell to which the PDSCH isassociated). In some embodiments, the mapping of the PDSCH data symbolsto the LTE time-frequency grid may also be adjusted to accommodate adifferently-sized control channel (e.g., to accommodate the fact that aneighboring cell uses three OFDM symbols for PDCCH, while the servingcell uses only two).

In support of this approach, appropriate signaling may be added, in someembodiments, to support dynamic adaptation of the mentioned PDSCHmapping. More specifically, signaling may be added to inform a receivingmobile station about the mapping used for a particular PDSCHtransmission. In other words, this additional signaling informs themobile station of which of several possible PDSCH mappings that themobile station should use when extracting PDSCH data symbols from theOFDM time-frequency grid and decoding the PDSCH. In some embodiments,this additional signaling could be part of the PDCCH, such as the partof the PDCCH carrying scheduling information for the mobile station.

FIGS. 4A and 4B provide a general illustration of this techniqueaccording to several possible embodiments of the invention. BS1corresponds to a first cell site and is the serving cell in each ofFIGS. 4A and 4B; mobile station 410 thus continuously monitors thecontrol channel, PDCCH, transmitted from BS1. On the other hand, PDSCHmay from time to time be transmitted from either BS1, as in FIG. 4A, orfrom BS2, at a neighboring cell site, as in FIG. 4B. (Of course, a givenbase station may include radio equipment and corresponding antennas formultiple cell sectors—for simplicity, only a single cell sector for eachbase station is discussed here.) In embodiments of the presentinvention, PDCCH carries control signaling that informs mobile stationas two which mapping is used for the current transmission of PDSCH.Thus, in FIG. 4A the PDCCH may indicate that a first mapping,corresponding to the mapping normally used by BS1, should be used whenextracting PDSCH data from the received signal and decoding the PDSCH.In FIG. 4B, however, the PDCCH may indicate that a second mapping,corresponding to the mapping normally used by BS2, should be usedinstead. As the transmission of the PDSCH may dynamically switch betweenBS1 and BS2, mobile station 410 dynamically changes its de-mappingprocess as well.

The additional signaling described above can be implemented in variousways. In one exemplary embodiment of the present invention, the PDCCHcontains a bit field describing the reference-signal frequency shiftthat the receiving mobile station should assume has been used in mappingthe PDSCH to the resource elements of the resource grid. In other words,this bit field indicates the mapping pattern that the mobile stationshould use when extracting user data from the time-frequency resourcegrid of the received signal. The size of this bit field may vary, insome embodiments, or be fixed, in others. For base stations configuredto use two or four cell-specific antenna ports, there are only threepossible reference signal shifts, and thus two bits would suffice. Onthe other hand, systems configured to use one cell-specific antennaports may need to use a control element comprising three bits. In eithercase, assuming a UE-specific reference signal is being used, theresource elements utilized in the PDSCH transmission can then be madecompatible with the site and/or sector from which the PDSCH transmissionis being performed, thus avoiding collisions between the PDSCH datasymbols and the cell-specific reference signals transmitted from thetransmitting cell site.

A similar approach may be used for taking the control-region size intoaccount for PDSCH transmissions using UE-specific reference signals.Thus, in some embodiments of the invention, the PDCCH contains anadditional control element that signals the number of OFDM symbols thatthe mobile station should assume for the control region when determiningto what set of resource elements the PDSCH has been mapped. In someembodiments, this additional control element may be jointly coded withthe control element that indicates the reference-signal frequency shift.Alternatively, this signaling (as well as the signaling indicating thereference signal shift) could be performed by means of higher layersignaling, such as Radio Resource Control (RRC) signaling.

With the preceding discussion of the application of the presentinvention to an LTE system in mind, those skilled in the art willappreciate that FIG. 5 illustrates a general method, such as may beimplemented in a wireless terminal, for receiving user data in awireless communication system that employs coordinated multi-pointtransmission of the user data from a first cell serving the wirelessterminal and a second cell site neighboring the first cell site. In thissystem, the first cell site maps control signals and user data totime-frequency resources according to a first mapping pattern, while thesecond cell site maps control data and traffic data to thetime-frequency resources according to a second mapping pattern. Thecontrol signals may include common reference signals, UE-specificreference signals, synchronization signals, and the like.

The illustrated method thus begins, as shown at block 510, with thereceiving of a transmission from the first cell site. This step mayinclude, of course, monitoring a control channel from the first cellsite to detect a specific allocation of resources. In the case of an LTEsystem, this allocation would designate specific resource blockstargeted to the mobile station.

As shown at block 520, the method continues with the extracting of userdata from the received data transmission according to the first mappingpattern, i.e., the pattern used by the first cell site to map the userdata and control data to the transmission. In some embodiments, themobile station may determine which mapping pattern should be used forthis particular transmission according to conventional methods, such asby determining a reference shift from the broadcast cell ID transmittedby the first cell site. In others, the mobile station may determinewhich mapping pattern should be used by detecting one or more specificcontrol elements, perhaps included in a downlink resource assignmenttransmitted by the first cell site. This control element (or elements)may index one of the several possible reference shifts, for example,and/or indicate how many OFDM symbols are dedicated to the downlinkcontrol channel.

As shown at block 530, the method continues with the receiving of atransmission from the second cell site. Those skilled in the art willappreciate that the mobile station need not “know” that thistransmission is coming from the second cell site, as the mobile stationcontinues to receive its downlink resource assignments from the controlchannel transmitted by the first cell site. However, as discussed indetail above, this transmission from the second cell site includestraffic data and control signals (such as reference signals) mapped tothe time-frequency resources according to the second mapping pattern,which differs from the first.

As shown at block 540, the mobile station learns that this is the caseby detecting a control element indicating that the second mappingpattern is currently used. As noted earlier, this control element mightbe included, for example, in one or more bits of a downlinkresource-allocation message, or elsewhere in the control channel. Insome embodiments, the control element indicates one of a plurality ofpre-determined shift patterns for common reference signals interspersedamong time-frequency resources mapped to user data. In some embodiments,the control element further indicates that the second mapping patternmaps user data to one or more fewer OFDM symbols than the first mappingpattern, e.g., because the second cell site dedicates more OFDM symbolsto the downlink control channel.

In some embodiments, this control element might only be used fortransmissions that are mapped according to a pattern other than the“normal” pattern. In other words, the mobile station might be able todetermine a default pattern, e.g., using the cell ID or other broadcastcontrol information. This default pattern could then be used for alltransmissions for which an additional control element is not received,e.g., for all transmissions from the serving cell site. In theseembodiments, the detection of the additional control element wouldsignal the mobile station that a second mapping pattern should be usedinstead. However, in other embodiments, this additional control elementcan be transmitted with every resource assignment, so that the mobilestation always determines the appropriate mapping pattern to use fromthe additional control element.

In any case, as shown at block 550, the process illustrated in FIG. 5continues with the extracting of user data from the transmissionreceived from the second cell site, according to the second mappingpattern. The illustrated procedure may be repeated as many times asnecessary, with dynamic switching between transmissions from the firstcell site and the second cell site as propagation conditions change, asthe mobile station moves, or as the interference environment changes.

Those skilled in the art will appreciate, then, that the techniquesdiscussed above facilitate the mapping of PDSCH (or similar)transmissions onto a time-frequency resource grid using a differentmapping pattern than used in the serving cell (i.e., the logical cell towhich the PDSCH is associated with). Similarly, the control-region sizemay also be assumed to be different. Those skilled in the art will alsoappreciate that this may be particularly beneficial for PDSCHtransmissions using UE-specific reference signals, since this allows thePDSCH to be transmitted from another cell site than normally used as theserving cell site, while completely avoiding collisions withtransmissions from the other cell site. Coordinated scheduling with fastsector selection could in this way be implemented without causing highinterference to mobile stations relying on cell-specific referencesignals. Likewise, the interference from the cell-specific referenceonto the mentioned PDSCH disappears. Thus, in various embodiments of themethod illustrated in FIG. 5, the first and second transmissions, fromthe first and second cell sites, respectively, are during non-coincident(i.e., non-overlapping) time intervals.

On the other hand, the techniques illustrated in FIG. 5 may also beapplied to COMP systems utilizing joint transmission, in which case thefirst and second transmissions discussed above may be at least partiallyoverlapping in time. In some of these embodiments, the mobile stationmay be configured to separate the first and second transmissions usingspace-time diversity processing, spatial de-multiplexing processing, orthe like.

For joint transmission from multiple cell sites (corresponding tomultiple logical cells) that use different reference signal frequencyshifts, reference-signal collisions with PDSCH are inevitable. However,the signaling of reference-signal frequency shift information for PDSCHmapping according to the above-described techniques provides the eNodeBwith the opportunity to select a shift, for a given PDSCH transmission,that causes the least amount of interference. For instance, perhaps oneof the cells in a COMP cluster does not have any mobile stations relyingon cell-specific reference signals for their PDSCH reception. In thiscase, using a reference-signal frequency shift corresponding to thatcell may then reduce the negative impact of the transmitted PDSCH toreference signals. Furthermore, the ability to dynamically signal anindicator of the control region size for the purpose of PDSCH mappingmay also be beneficial in that the control-region size may vary from onecell site to another within the COMP cluster, while the PDSCH mappingfor the mobile stations participating in the COMP transmission could beassuming a control region size equal to the maximum control region sizecurrently used within the COMP cluster.

Finally, the above mentioned PDSCH mapping flexibility could be used inconjunction with COMP based on cell-specific reference signals as well.However, for that case, the mobile station may need to know the cell IDof the cell sites participating in the COMP transmission, in order toestimate the channels. Thus, in these cases the reference-signalfrequency shift that the mobile station should use in de-mapping PDSCHtransmissions from the downlink time-frequency grid may alternatively besignaled implicitly via the cell ID, while an indication of thecontrol-region size would still be explicitly signaled unless it can beassumed that the mobile station is able to decode the control-channelformat indicator (PCFICH, in LTE systems) giving the control-region sizeof the non-serving cell sites in the COMP cluster.

Those skilled in the art will appreciate that the techniques describedabove, although generally described in the context of an LTE system, maymore generally be implemented in a variety of wireless apparatus adaptedfor use in a wireless communication system that employs coordinatedmulti-point transmission. FIG. 6 is a schematic diagram illustrating thefunctional components of such a system, including a transmitting node610 connected to a cell site 630, and a wireless terminal 635.

Transmitting node 610 includes a receiver section 615 and transmittersection 620, each of which may be design, according to well-known designtechniques, for compatibility with one or more wireless standards (suchas LTE). Transmitting node 610 further includes a controller section625, which again may be designed, according to well-known designtechniques, to implement one or more wireless standards. In particular,controller 625 may be configured to map user data to downlinktime-frequency resources according to a first mapping pattern, for atransmission to wireless terminal 635. However, controller 625, whichmay include one or more microprocessors or the like, configured withappropriate software and/or firmware, may further be configured totransmit an additional control element to the wireless terminal,specifically indicating that user data associated with the controlelement is mapped to the time-frequency resources according to the firstmapping pattern. As noted above, this control element may be included inor associated with a downlink resource allocation message, such thattransmitting the control element comprises transmitting the downlinkresource allocation message.

Like transmitting node 610, wireless terminal 635 may be designed,according to well-known design techniques, for compatibility with one ormore wireless standards, such as LTE. Thus, in some embodiments,wireless terminal 635 includes a duplexer 650, a radio-frequency (RF)receiver (RX) section 645, and RX baseband circuit 640, each of whichmay generally be configured according to conventional means. However, RXbaseband circuit 640 is further configured, according to someembodiments of the invention, to carry out one or more of the inventivetechniques described above. In particular, RX baseband circuit 640 isconfigured, in several embodiments of the invention, to extract userdata according to a first mapping pattern from time-frequency resourcesof a first transmission for the wireless terminal 635 transmitted from afirst cell site, to detect a control element transmitted by one of thefirst and second cell sites, the control element indicating that userdata associated with the control element is mapped to the time-frequencyresources according to a second mapping pattern, and, in response todetecting the control element, to extract user data according to thesecond mapping pattern from time-frequency resources of a secondtransmission for the wireless terminal 635 transmitted from a secondcell site.

FIG. 7 provides some details of an exemplary receiver circuit 640,including one or more processors 710 (which may include one or moremicroprocessors, microcontrollers, digital signal processors, or thelike) and other digital hardware 720 (including, for example, customizedsignal processing logic). Either or both of processors 710 and otherdigital hardware 720 may be configured with software and/or firmwarestored in memory 730. In particular, this software includes receiverprocessing code 740, which comprises instructions for carrying out oneor more of the techniques described above. Memory 730 may also includeother processing code (not shown), as well as program data 746,configuration data 748, and other control data 749, some of which may bestored in a random-access memory (RAM) or flash memory.

More generally, those skilled in the art will appreciate that receivercircuit 640 may comprise any of a variety of physical configurations,such as in the form of one or more application-specific integratedcircuits (ASICs). Other embodiments of the invention may includecomputer-readable devices, such as a programmable flash memory, anoptical or magnetic data storage device, or the like, encoded withcomputer program instructions which, when executed by an appropriateprocessing device, cause the processing device to carry out one or moreof the techniques described herein for equalizing received signals in acommunications receiver.

The present invention may, of course, be carried out in other ways thanthose specifically set forth herein without departing from essentialcharacteristics of the invention. The present embodiments are to beconsidered in all respects as illustrative and not restrictive, and allchanges coming within the meaning and equivalency range of the appendedclaims are intended to be embraced therein.

What is claimed is:
 1. A method in a wireless terminal for receivinguser data in a wireless communication system enabling coordinatedmultipoint transmission of the user data from a first cell site servingthe wireless terminal and a second cell site neighboring the first cellsite, wherein the first cell site maps control signals and user data toa plurality of time-frequency resources according to a first mappingpattern and the second cell site maps control signals and user data tothe plurality of time-frequency resources according to a second mappingpattern, the method comprising: extracting user data, according to thefirst mapping pattern, from time-frequency resources of a firsttransmission for the wireless terminal transmitted from the first cellsite; detecting a control element transmitted by the first cell site,the control element indicating that user data associated with thecontrol element is mapped to the time-frequency resources according tothe second mapping pattern; and responsive to said detecting, extractinguser data according to the second mapping pattern from time-frequencyresources of a second transmission for the wireless terminal transmittedfrom the second cell site.
 2. The method of claim 1, wherein the controlsignals comprise one or more of control-channel data, cell-specificreference signals, user-equipment-specific reference signals, andsynchronization signals.
 3. The method of claim 1, wherein detecting thecontrol element comprises decoding one or more bits of a receiveddownlink resource-allocation message.
 4. The method of claim 1, whereinthe control element indicates one of a plurality of pre-determined shiftpatterns for common reference signals interspersed among thetime-frequency resources mapped to user data.
 5. The method of claim 1,wherein the control element further indicates that the second mappingpattern maps user data to one or more fewer OFDM symbols than the firstmapping pattern.
 6. The method of claim 1, wherein the first and secondtransmissions occur during first and second non-coincident transmissiontime intervals.
 7. The method of claim 1, wherein the first and secondtransmissions at least partially overlap in time, and wherein the methodfurther comprises separating the first and second transmissions usingone of space-time diversity processing and spatial de-multiplexingprocessing.
 8. The method of claim 1, wherein the control signalscomprise user-equipment-specific reference signals interspersed amongthe time-frequency resources mapped to user data according to the secondmapping pattern, and wherein the method further comprises extracting theuser-equipment-specific reference signals from the second transmissionaccording to the second mapping pattern.
 9. A wireless terminal for usein a wireless communication system enabling coordinated multipointtransmission of user data from a first cell site serving the wirelessterminal and a second cell site neighboring the first cell site, whereinthe first cell site maps control signals and user data to a plurality oftime-frequency resources according to a first mapping pattern and thesecond cell site maps control signals and user data to the plurality oftime-frequency resources according to a second mapping pattern, thewireless terminal comprising a receiver circuit configured to: extractuser data according to the first mapping pattern from time-frequencyresources of a first transmission for the wireless terminal transmittedfrom the first cell site; detect a control element transmitted by thefirst cell site, the control element indicating that user dataassociated with the control element is mapped to the time-frequencyresources according to the second mapping pattern; and in response todetecting the control element, extract user data according to the secondmapping pattern from time-frequency resources of a second transmissionfor the wireless terminal transmitted from the second cell site.
 10. Thewireless terminal of claim 9, wherein the control signals comprise oneor more of control-channel data, cell-specific reference signals,user-equipment-specific reference signals, and synchronization signals.11. The wireless terminal of claim 9, wherein the receiver circuitdetects the control element by decoding one or more bits of a receiveddownlink resource-allocation message.
 12. The wireless terminal of claim9, wherein the control element indicates one of a plurality ofpre-determined shift patterns for cell-specific reference signalsinterspersed among the time-frequency resources mapped to user data. 13.The wireless terminal of claim 9, wherein the control element furtherindicates that the second mapping pattern maps user data to one or morefewer OFDM symbols than the first mapping pattern.
 14. The wirelessterminal of claim 9, wherein the first and second transmissions occurduring first and second non-coincident transmission time intervals. 15.The wireless terminal of claim 9, wherein the first and secondtransmissions at least partially overlap in time, and wherein thereceiver circuit is further configured to separate the first and secondtransmissions using one of space-time diversity processing and spatialde-multiplexing processing.
 16. The wireless terminal of claim 9,wherein the control signals comprise user-equipment-specific referencesignals interspersed among the time-frequency resources mapped to userdata according to the second mapping pattern, and wherein the receivercircuit is further configured to extract the user-equipment-specificreference signals from the second transmission according to the secondmapping pattern.
 17. A method for transmitting user data, in atransmitter node of a first cell site in a wireless communication systemenabling coordinated multipoint transmission of user data from the firstcell site and a second cell site serving a wireless terminal andneighboring the first cell site, the method comprising: mapping userdata, according to a first mapping pattern, to time-frequency resourcesof a first transmission for the wireless terminal transmitted from thefirst cell site; and transmitting a control element indicating that userdata associated with the control element is mapped to the time-frequencyresources according to a second mapping pattern.
 18. The method of claim17, further comprising including the control element in a downlinkresource allocation message, wherein transmitting the control elementcomprises transmitting the downlink resource-allocation message.
 19. Atransmitting node for use in a first cell site in a wirelesscommunication system, the wireless communication system enablingcoordinated multipoint transmission of user data from the first cellsite and a second cell site serving a wireless terminal and neighboringthe first cell site, the transmitting node comprising a transmittercircuit configured to: map user data, according to a first mappingpattern, to time-frequency resources of a first transmission for thewireless terminal transmitted from the first cell site; and transmit acontrol element indicating that user data associated with the controlelement is mapped to the time-frequency resources according to a secondmapping pattern.
 20. The transmitting node of claim 19, wherein thetransmitter circuit is further configured to include the control elementin a downlink resource allocation message and to transmit the controlelement by transmitting the downlink resource-allocation message.